Diffraction Pattern Of Optical Tweezers And Its Application To Particle Manipulation

With the wide application and development of optical tweezers technology,the researching interesting has been focused on the number and types of particles that can be manipulated by optical tweezers.Traditional optical tweezers can easily manipulate a single or a few transparent colloidal particles,but a more effective and simple method is needed to trap and manipulate multiple colloidal particles simultaneously.Furthermore,the absorbing particles have a greater absorption capacity for the laser,which will produce larger scattering forces and thermal effects near the focal point,making it difficult for the absorbing particles to be trapped stably.Therefore,it is necessary to find an effective method to manipulate the absorbing particles.Generally,the optical tweezers use the zero order diffraction spot of the focused beam to exert a small force on the particles to achieve the manipulation of the particles.When the power of trapping laser is low,the laser power of the high-order diffraction fringes is very weak and the corresponding optical force can be ignored.But as the power increases,this optical force cannot be ignored.In this thesis,the optical trap force,generated by the high-order diffraction fringes of the trapping laser beam,is used to trap multiple transparent particles simultaneously.Furthermore,three-dimensional oscillation of the absorbing particles is realized near the center of the optical trap.First,this paper uses the diffraction pattern of the optical tweezers system to trap and arrange multiple particles at the same time.The impact factors on the trapped particle patterns have been investigated,such as the trapped light source,particle's concentration and size.Second,the optical tweezers can trap multiple particles simultaneously,which can easily cause blockage of the sample cell when the sample cell is with small size.In this thesis,to eliminate the aggregation of particles,an effective method is proposed by using the total reflection interface as the substrate of the sample cell.Third,when the optical tweezers are applied to manipulate the absorbing particles,the particles are usually pushed away and no longer return to the optical trap center.In this thesis,the particles are pushed back to the optical trap center under the influence of the optical force of the diffraction fringes,and three-dimensional oscillations are performed close to the substrate of the sample cell.In addition,by adjusting the thickness of the sample cell to adjust the thermal convection force,stable trapping of the absorbing particles can be achieved at the edge of the focused spot.